The IEEE (Institute of Electrical and Electronics Engineers) 802.16 standards propose using an Orthogonal Frequency Division Multiple Access (OFDMA) for transmission of data over an air interface. OFDMA has also been proposed for use in 3GPP (Third Generation Partnership Project) Evolution communication systems. In an OFDMA communication system, a frequency bandwidth is split into multiple contiguous frequency sub-carriers, wherein groups of sub-carriers are arranged in logical frequency sub-bands (not necessarily contiguous in frequency), each sub-band comprising multiple orthogonal frequency sub-carriers, that are transmitted simultaneously. A user may then be assigned one or more of the frequency sub-bands for an exchange of user information, thereby permitting multiple users to transmit simultaneously on the different sub-bands. These sub-bands are orthogonal to each other, and thus inter-user and intra-cell interference is minimized.
In order to provide more efficient use of the channel bandwidth, a radio access network (RAN) may transmit the data using multiple antennas and a user equipment (UE) may receive the transmitted data using multiple receiving antennas, referred to as Multiple Input-Multiple Output (MIMO). In an OFDMA system that implements MIMO, a serving RAN may beamform a downlink signal for transmission to each UE via an antenna array and over an associated sub-band. In order to beamform the signal, the RAN maintains a set of (transmit) weights in association with each UE and each element of the antenna array. When the RAN transmits to the UE, the RAN applies an appropriate weight, of the set of weights, to the signal applied to each element of the array. In order to determine the set of weights for each UE, the RAN measures uplink channel conditions in association with the UE. That is, for any given measuring period, such as a Transmission Time Interval (TTI) (also known as a sub-frame), a UE served by the RAN transmits a pre-determined symbol to the RAN in a sub-band allocated to the UE by the RAN. Based on a comparison of the symbol received to the symbol that the RAN knows was transmitted, the RAN is able to estimate channel conditions for the UE in the allocated sub-band and determine a set of weights for a downlink transmission to the UE in the sub-band.
For example, FIG. 1 is a block diagram 100 depicting a channel sounding of a frequency bandwidth 102 in accordance with the prior art. As depicted in FIG. 1, during a first transmission interval 104 a RAN transmits a first downlink (DL) sub-frame 110. During a next, second transmission interval 106 a UE served by the RAN transmits an uplink (UL) sub-frame 120 to the RAN, and during a next, third transmission interval 108 the RAN transmits a second DL sub-frame 130. Between each sub-frame is a transition time interval, or gap 150, 152. More particularly, between DL sub-frame 110 and UL sub-frame 120 is a Transmit Transition Gap (TTG) 150 and between UL sub-frame 120 and DL sub-frame 130 is a Receive Transition Gap (RTG) 152. During these gaps, the UEs and RANs are not transmitting and are changing from a transmit or a receive mode to a receive or a transmit mode. Typically, these gaps are of a length corresponding to a round trip time delay to an edge of a coverage area of the RAN and a processing time delay involved in the UE or RAN processing a received message and switching modes.
Each DL sub-frame 110, 130 includes a DL scheduling field (DL-MAP) 114, 134, an UL scheduling field (UL-MAP) 116, 136, and a DL data packet field 118, 138. Each DL sub-frame 110, 130 further may include a preamble field 112, 132. DL scheduling field 114, 134 provides a frame duration, a frame number, a DL sub-band allocation for DL bursts, and a coding and modulation scheme used for each DL burst. UL scheduling field 116, 136 provides UL sub-band scheduling for UL bursts, a coding and modulation scheme used for each UL burst, and a start time for each UL burst. DL data packet field 118, 138 comprises the DL bursts, that is, is the field in which the RAN transmits data packets to the served UEs based on the sub-band scheduling and the determined beamforming weights. Preamble field 112, 132 typically comprises pilots that may be used by UEs for timing synchronization, frequency synchronization, and channel estimation.
UL sub-frame 120 includes an UL data packet field 122 and a sounding zone 124. UL data packet field 122 comprises UL bursts, that is, is the field in which the UEs transmit data packets to the RAN based on UL scheduling field 116. Sounding zone 124 is a field in which each of one or more UEs served the RAN transmits, over the frequency carriers allocated to the UE, a predetermined OFDM symbol known to both the RAN and the UE. Channel sounding assumes a reciprocity of the UL and DL channels and also assumes the RAN has a means of accounting for any non-reciprocities that may exist in the RAN transceiver hardware. Based on the received symbol the RAN is then able to determine a RAN-to-UE channel response. For example, as depicted in FIG. 1, a UE may transmit an OFDM symbol, that is, a known waveform, to a serving RAN in sounding zone 124 of UL frame 120, during time interval 106, and over designated sub-carriers of the frequency bandwidth 102. Based in the received symbol, the RAN is able to estimate a RAN-to-UE channel response, schedule a sub-band comprising a set of sub-carriers 126 for a downlink transmission to the UE, and determine a set of weights for the DL transmission to the UE over the scheduled set of sub-carriers. The RAN then conveys a DL burst 140 to the UE in a DL data packet field 138 of DL sub-frame 130 transmitted during the next time interval 108. The DL burst is transmitted over the scheduled set of sub-carriers and sub-band using the set of weights determined based on the received sounding zone symbol.
A drawback of channel sounding is that it consumes a significant overhead. That is, channel sounding takes up a symbol in an UL sub-frame for each reporting UE. In OFDMA communication systems, where the frequency domain consists of many sub-bands, each sub-band may require a channel feedback. Therefore, providing such feedback, especially in closed-loop MIMO systems where feedback is needed for beamforming DL transmissions to each served UE, can be significant.
Accordingly, there is a need for a method and apparatus for an OFDMA system that provides an improved channel sounding design.